In general, a secondary battery is configured as an electrode assembly composed of a cathode, an anode and a separator interposed between the cathode and the anode, of a stacked or wound configuration, a battery casing made of a metal can or a laminate sheet, and an electrolyte filled in the battery casing. The recently developed lithium secondary battery is widely used in electronic devices such as cellular phones, notebook computers, electric vehicles, etc.
The secondary battery is classified into a liquid electrolyte battery and a polymer electrolyte battery depending on the electrolyte used. In general, a battery using a liquid electrolyte is called a lithium ion battery and a battery using a polymer electrolyte is called a lithium polymer battery. The secondary battery is prepared into various shapes, including cylindrical, prismatic and pouch-type shapes.
In general, a secondary battery includes a can as a battery casing, an electrode assembly accommodated inside the can, a cap assembly coupled to an open upper end of the can, a lead for electrically connecting an anode or cathode terminal to a protection circuit and a washer coupled to an upper side of a cap plate. However because the washer is located at the upper side of the cap plate, an electrolyte injection hole is covered by the washer. For this reason, the washer has to be removed to check if there is leakage of the electrolyte. In addition, when a trace amount of the electrolyte leaks, the leakage is not easily detected with naked eyes outside of the battery and the electrolyte leakage goes unnoticed.